Use the Kiosk howto to install a R/O Kiosk. Magic Mirror software runs from the home directory of the user and can only be R/W. I used a USB SSD connected to the USB3 port of the Raspberry Pie 4.

Make a partition on the USB disk and format it as EXT4. Then use blkid to see PARTUID:

blkid
/dev/mmcblk0p1: LABEL_FATBOOT="boot" LABEL="boot" UUID="5203-DB74" TYPE="vfat" PARTUUID="6c586e13-01"
/dev/mmcblk0p2: LABEL="rootfs" UUID="2ab3f8e1-7dc6-43f5-b0db-dd5759d51d4e" TYPE="ext4" PARTUUID="6c586e13-02"
/dev/sda1: UUID="2f7dbca4-e910-4517-b70a-efad86e0b8d3" TYPE="ext4" PARTUUID="b3a803bf-01"

In this case the PARTUUID of /dev/sda1 is b3a803bf-01. Change /etc/fstab to look something like this:

/etc/fstab

proc                  /proc              proc    defaults                                    0 0
PARTUUID=6c586e13-01  /boot              vfat    defaults,ro                                 0 2
PARTUUID=6c586e13-02  /                  ext4    defaults,noatime,ro                         0 1
PARTUUID=b3a803bf-01  /home              ext4    defaults,noatime                            0 0
tmpfs                 /tmp               tmpfs   defaults,noatime,nosuid,mode=1777,size=100m 0 0
tmpfs                 /var/tmp           tmpfs   defaults,noatime,nosuid,size=30m            0 0
tmpfs                 /var/log           tmpfs   defaults,noatime,nosuid,mode=0755,size=100m 0 0
tmpfs                 /var/run           tmpfs   defaults,noatime,nosuid,mode=0755,size=2m   0 0
tmpfs                 /etc/console-setup tmpfs   defaults,noatime,nosuid,mode=0755,size=2m   0 0
tmpfs                 /var/lib/chrony    tmpfs   defaults,noatime,nosuid,mode=0755,size=2m   0 0
tmpfs                 /var/lib/lightdm  tmpfs   defaults,noatime,nosuid,size=30m            0 0

Reboot and see if /home is mounted. Create a directory for the user pi

mkdir /home/pi
chmod 700 /home/pi
chown pi. /home/pi

Now we are going to undo the changes of the system wide autostart and put them in the homedir of the pi user. Make it look like it was:

/etc/xdg/openbox/autostart

#
# These things are run when an Openbox X Session is started.
# You may place a similar script in $HOME/.config/openbox/autostart
# to run user-specific things.
#

# If you want to use GNOME config tools...
#
#if test -x /usr/lib/arm-linux-gnueabihf/gnome-settings-daemon >/dev/null; then
#  /usr/lib/arm-linux-gnueabihf/gnome-settings-daemon &
#elif which gnome-settings-daemon >/dev/null 2>&1; then
#  gnome-settings-daemon &
#fi

# If you want to use XFCE config tools...
#
#xfce-mcs-manager &

Now create the autostart file for the pi user as the pi user

cd ~
mkdir -p .config/openbox

/home/pi/.config/openbox/autopstart

xset -dpms
xset s off
xset s noblank
unclutter -idle 0 &
cd ~/MagicMirror
xrandr --output HDMI-1 --rotate left &
npm start &

Rotate the graphical interface does not seem to work on a Pi 4. As you can see we are using xrandr to rotate. Rotation of the console must be disabled and the GL driver must be enabled. Use raspi-config to change to GL driver: 7 Advanced Options → A8 GL driver → G2 GL (Fake KMS) Then change /boot/config.txt to remove rotate screen

/boot/config.txt

.
.
.
[pi4]
# Enable DRM VC4 V3D driver on top of the dispmanx display stack
dtoverlay=vc4-fkms-v3d
max_framebuffers=2

[all]
dtoverlay=vc4-fkms-v3d

#display_rotate=3
disable_splash=1